Flexible pipes of the present type for offshore transportation of fluids are well known in the art and are for example described in “Recommended Practice for Flexible Pipe”, API 17 B, second Edition, 1998, which provides a standard for such pipes. Such pipes usually comprise an inner liner also often called an inner sealing sheath or an inner sheath, which forms a barrier against the outflow of the fluid which is conveyed through the pipe, and one or more armouring layers on the outer side of the inner liner (outer armouring layer(s)). An outer sealing sheath may be provided with the object of forming a barrier against the ingress of fluids from the pipe surroundings to the armour layers.
Typical unbonded flexible pipes are e.g. disclosed in WO0161232A1, U.S. Pat. No. 6,123,114 and U.S. Pat. No. 6,085,799.
The term “unbonded” means in this text that at least two of the layers including the armouring layers and polymer layers are not bonded to each other. In practice the known pipe normally comprises at least two armouring layers located outside the inner sealing sheath. These armouring layers are not bonded to each other directly or indirectly via other layers along the pipe. Thereby the pipe becomes bendable and sufficiently flexible to roll up for transportation.
During use of a flexible pipe for transportation of hydrocarbon containing and/or water containing fluids, gasses such a CO2, H2S and H2O will often migrate through the inner sealing sheath and into an annular space, also called an annulus, outside the inner sealing sheath provided by the inner sealing sheath and an additional sealing sheath surrounding the inner sealing sheath such as an outer sealing sheath. Over time the pressure in such annulus may become quite considerable, and in order to prevent such pressure from damaging the pipe, different structures have been provided to allow the penetrated gasses to escape from the annulus.
EP 1 119 684 discloses such flexible pipe construction where gasses penetrated from the bore of the pipe into the annulus are conveyed away via the bore by providing a passageway with a bypass valve. WO 2008/053142 describes a flexible pipe comprising a bore, an inner sealing sheath and an annulus outside the inner sealing sheath where the annulus comprises a passage for venting the annulus in order to reduce the risk of damaging of the pipe, where the passage can be provided in a terminating end fitting and be provided with a measuring device for monitoring the pressure or other parameters of the gas.
However, the inventor of the present invention has found the even though the penetrated gas is allowed to escape to avoid overpressure, the penetrated gas has other damaging effects, and the purpose of the present invention is to alleviate such damaging effects.
Accordingly it has been found that even though the penetrated gas is allowed to escape to avoid overpressure, the penetrated gas causes an undesired corrosion of armouring elements provided in the annulus and in general such penetration of gasses has shown to reduce the durability of the pipe.
The present invention provides an unbonded flexible pipe in which corrosion of armouring elements provided in the annulus can be kept at a minimum.